464 



NATURE 



[February i, 19 12 



vided. The diameter of the can was 77 cm., and its axial 

 len{<th 6-6 rm. One end of this was provid»'d with .1 

 flexible diaphragm of phosphor bronze, whi( h could Iw 

 struck suddenly by the plunger of an elerlronjagnet. 'I h<- 



Fig. 1. — Apparatus for the production and photography of vortex rings 



Other end of the can could be closed with metal discs, 

 which were provided with one or more holes of various 

 shapes and dimensions. For the production of a single 



To make the rings visible, and at the tame time prr>'. 

 for keeping the water clear so that the tank would 

 havi- to b*' rep<-at«Klly refilled, it was necessary to fill 

 I .HI with hit^lilv . oloured water, the colouring of wt 

 \sould entirely disappear when the i 

 l.rokp up and dissipated. Among «.! 

 ! nring materials tried in the exji 

 . that which was most used 



\" )1 phthaline. The wat«r in thr- 



was made strongly alkaline, and 

 water in the tank was made sli|^ 

 ;icid. Thus the projected rings wer^ 

 ;i de«'p red colour, nnd entirely o 

 .ipjieared upon i up. An illus- 



tration IS hen I (Fig, i) of 



th" entire oiiim . ix.vc d«*scrib»^, 

 tof^ithcr with the plate drop and other 

 'I'vices employed in the photo-'r-inhing 

 of the rings. . 



Kxperiments prelimin.^ the 



photographic work revealed ihc fullow- 

 inf^ prop-rties of these rings. The 

 motion through the water is rapid. 

 Ixing aix)ut 2 metres per second. The 

 rings move in a straight path with a 

 )4ra(lually diminishing speed, and 

 nlways maintain their plane perpien- 

 dicular to their line of motion. When 

 tile water of the tank contains sus- 

 prnded particles of slightly greater 

 (Irnsity than water, a ring will pass 

 through and among them without in- 

 corporating and carrying any of the 

 particles along with it. When a piece 

 nf chiffon cloth is stretched over a frame 

 and is held in the path of a ring, the 

 ring will be found to pass easily through 

 it without being broken up or much 

 disturbed in its motion. When a piece 

 of tissue paper similarly stretched on a 

 frame is held in the path of the ring, 

 the ring will break through it, though 

 it is in turn broken up by the impact. If a light watch- 

 chain hangs in the water and is fairly struck by a ring it 

 is bent into a decided curve by the force of the blow. The 



Fig. 2.— Stereoscopic photographs of a vortex ring just issuing from the gun. 



ring one hole in the centre of a disc was used i cm. in 

 diameter, and for the production of double rings two holes, 

 one above the other, each 085 cm., were employed. We 

 shall hereafter call this can the " tjuii.' 



NO. 2205, VOL. 88] 



kinetic energy of these rings is thus seen to be consider- 

 able. When two rings are made to approach from opposite 

 ends of the tank, they will on impacting be broken up anl 

 dissipated if they meet fairly, but if their line of approad. 



